The present invention relates to an internal magnetic circuit having a magnet, a yoke, and a plate, and a loudspeaker system incorporating the magnetic circuit.
The present application claims priority from Japanese Patent Application No. 2002-178329, the disclosure of which is incorporated herein by reference.
FIG. 1 shows one prior art example of a loudspeaker system incorporating an internal magnetic circuit disclosed in Japanese Patent Laid-Open Publication No. Hei 6-261393, and FIG. 2 is an explanatory view showing the internal magnetic circuit. A vertically magnetized magnet 2 and a plate 1 forming a pole piece are arranged inside a yoke 3 having a bottom part 3a and a side part 3b surrounding the plate 1. The magnet 2 is attached to the bottom face of the plate 1 and to the bottom part 3a of the yoke 3. An air gap or magnetic gap G is formed between the plate 1 and the yoke 3. The magnet 2, the yoke 3 on the side of one magnetic pole of the magnet 2, the plate 1 on the side of the other magnetic pole of the magnet 2, and the magnetic gap G form an internal magnetic circuit.
A voice coil bobbin 5 is arranged such as to surround the pillar-like plate 1 which forms the pole piece, so that a voice coil 4 is positioned inside the magnetic gap G. The voice coil bobbin 5 is supported on a frame 6 through a damper 7. The inner end of a diaphragm 8 is fixedly attached to the periphery of the voice coil bobbin 5, while the outer end of the diaphragm 8 is supported on the periphery of the frame 6 through an edge 8A. Reference numerals 9 and 10 represent a center cap and a gasket, respectively.
It is known that in such a loudspeaker system the winding width of the voice coil 4 relative to the effective length of the magnetic gap G has a close correlation with the distortion caused by the nonlinearity of drive force. Accordingly, a short voice coil design in which the voice coil 4 has a small winding width has been adopted so that even a maximum amplitude of the loudspeaker system does not cause the voice coil 4 to come out of the range of effective length of the magnetic gap G, whereby drive force variations in response to the input signal current are suppressed, and thus nonlinear distortion is prevented.
This short voice coil design has the effect of preventing nonlinear distortion on condition that the magnetic flux distribution in the magnetic gap G is uniform. In a conventional internal magnetic circuit, however, the magnetic flux density distribution inside the magnetic gap is not necessarily uniform particularly if it has a long effective length.
FIG. 2 shows the magnetic circuit formed by the magnet 2, the yoke 3 and plate 1 arranged on the opposite sides of the magnet 2, and the magnetic gap G. As shown, most of the magnetic flux lines form loops leaving from one magnetic pole of the magnet 2, passing through the bottom part 3a and side part 3b of the yoke 3, crossing the magnetic gap G, passing the plate 1, and entering the other magnetic pole of the magnet. The density of these magnetic flux lines tends to be high on the side of shorter loops, i.e., the nearer the loops are to the magnet 2, the higher the density is, and vice versa.
That is, within the effective length X0–X1 of the magnetic gap G, the magnetic flux density is higher on the X0 side, while it is lower on the X1 side. The magnetic flux density decreases in the direction of from X0 to X1 as shown in FIG. 3 within the effective length X0–X1, meaning that the magnetic flux density is not uniform in the direction in which the voice coil 4 moves.
Therefore, large amplitude vibration of the voice coil 4 located inside the magnetic gap G resulting from a large input signal may lead to the nonlinear distortion of sound signals. This is particularly evident in a loudspeaker system with the aforementioned short voice coil design.